Route planning devices are well known in the field of navigational instruments. The method of route planning implemented by known prior art systems depends on the capabilities of system resources, such as processor speed and the amount and speed of memory. As increased system capability also increases system cost, the method of route planning implemented by a navigation device is a function of overall system cost.
One type of navigational system includes Global Positioning Systems (GPS). Such systems are known and have a variety of uses. In general, GPS is a satellite-based radio navigation system capable of determining continuous position and velocity information for an unlimited number of users. Formally known as NAVSTAR, the GPS incorporates a plurality of satellites which orbit the earth in extremely precise orbits. Based on these precise orbits, GPS satellites can relay their location to any number of receiving units.
The GPS system is implemented when a device specially equipped to receive GPS data begins scanning radio frequencies for GPS satellite signals. Upon receiving a radio signal from a GPS satellite, the device can determine the precise location of that satellite via one of different conventional methods. The device will continue scanning for signals until it has acquired at least three different satellite signals. Implementing geometrical triangulation, the receiver utilizes the three known positions to determine its own two-dimensional position relative to the satellites. Additionally, acquiring a fourth satellite signal will allow the receiving device to calculate its three-dimensional position by the same geometrical calculation. The positioning and velocity data can be updated in real time on a continuous basis by an unlimited number of users.
Further, with a navigational aid device cartographic data are loaded into a memory of the device and manipulated to provide route planning to a user of the device. Cartographic data can include by way of example only, thoroughfare identifications, intersection identifications, altitude information, longitude information, latitude information and the like. The cartographic data are voluminous and as a result, often only specific cartographic data associated with predefined geographic regions are loaded into the device during any particular operation cycle.
Using the cartographic data the device displays a portion of the data as a map to a user of the device, typically identifying the device's location and orientation within the displayed map. Often, the user requests a route within the displayed map which includes a generated path from the device's present location and orientation to a desired location. Accordingly, the route is derived from the cartographic data and presented to the user of the device. Presentation of the device's changing location with respect to the route continues in real time as the device travels along the route.
The process of plotting the device's present location, within the cartographic data, and mapping that location to the map is referred to as map matching or road locking. Generally, problems occur with the road locking process when the precise location of the device at any particular moment in time and space is inaccurate, or when the cartographic data contain slight inaccuracies.
Navigational systems can sometimes provide inaccurate information to a navigational device such that the device inaccurately calculates the precise location of the device. Typically, inaccurate information results when the device is traveling at a rate less than five miles per hour or when satellite interference occurs. Moreover, even when a device is traveling at rate above 5 miles per hour and there exists no satellite interference, the precise location of the device is still a calculated projection which is made by the device, and the projection is not without error. For example, a device traveling at a particular rate of speed having a particular angular direction will determine its location by rapidly calculating at least three locations for the device and then generating a fourth likely location which is road locked to the map.
However, at any particular moment the device's location can coincide with thoroughfare choices emanating from the route, such that the error margin in determining the device's precise location when compared with the now available thoroughfare locations, results in road locking the location of the device to a location off the route. This problem is particularly noticeable when cartographic data associated with the available thoroughfare locations, which are off the route, vary only slightly with available thoroughfare locations, which are on the route. Moreover, these variations within the cartographic data for the thoroughfares can be the result of erroneous cartographic data.
For example, consider two thoroughfare choices, one on the route and one off the route. But, each thoroughfare runs parallel to the other and is separated by only a concrete divider and each thoroughfare runs in parallel to the other for a distance in excess of one mile. A calculation to retrieve the device's location results in road locking the device's location to the thoroughfare located off the route, when in fact the device is located on the route. Obviously, a user of the device will quickly become frustrated and develop a perception that the performance of the device is malfunctioning.
Clearly, in many cases halting travel is not a viable alternative. For example, when the user is traveling on an interstate it is entirely impossible to simply stop. The alternative of pulling off on the shoulder is undesirable and can be dangerous. Pulling off on an exit is equally undesirable since doing so increases travel time and provides an added inconvenience to the user. In other instances, such as navigating downtown city streets, the traffic issues alone may prevent the user from stopping their vehicle during the recalculation process. Even if the user has the ability to safely stop their vehicle, such as when traveling in a neighborhood, the closeness in proximity of available thoroughfares can still yield an inaccurate road lock. Accordingly, capabilities to favorably road lock the location of the device to a thoroughfare located on the route is desirable and is also often a correct reflection of the device's precise location. To achieve this result, more efficient map matching capabilities are needed.
In summary, current prior art systems do not provide adequate map matching or road locking capabilities. Further, as users demand products with greater accuracy and usability, the problem will continue to escalate. As a result, present devices which inadequately perform map matching often frustrate users when thoroughfare choices result in inaccurate calculations that assume the devices' locations are off the route, when in fact the locations of the devices are on the route.
Therefore, there exists a need for a navigational device which more accurately performs map matching capabilities than current systems In addition, there is also a need for a navigational route planning device which efficiently maps a device's current position to a planned position on a route.